For devices that emit radio waves, acceptable limits of radio waves are defined in consideration of the effects of radio waves on the human body.
As a criterion for determining the limits, specific absorption rate (SAR) is used. SAR indicates an amount of energy absorbed per unit mass of tissue per unit time, and is expressed in units of W/kg. A greater SAR value indicates greater effects on the human body.
For example, in the United States, the upper limit of the energy amount absorbed by any 1 gram of human tissue in six minutes is 1.6 W/kg. Among devices that emit radio waves, particularly wireless routers are subject to stricter standards for SAR measurement than mobile telephones and the likes. For example, SAR measurement locations for wireless routers are defined to be closer to the subject devices than the case of usual mobile telephones.
Hence, even in the case of mobile telephones, when they have a function of wirelessly connecting to an external device and a base station and relaying data between the external device and the base station, such mobile telephones, particularly while relaying data, are required to meet the condition that the SAR value measured at a closer location is at or below the aforementioned limit.
In Japan, devices that emit radio waves need to be designed to meet the condition that the upper limit of the energy amount absorbed by any 10 grams of human tissue in six minutes is 2.0 W/kg.
Here, since a SAR value increases with increased transmission power, it is possible that mobile telephones having the relaying function are given a lower upper limit of transmission power than usual mobile telephones.
As technology of lowering the maximum value of transmission power, technology disclosed in Non-Patent Literature 6 is well known, for example. As general technology of lowering the SAR value, technology disclosed in Patent Literature 1 is well known, for example.